Visible Light Communication (VLC), which is a typical illumination-communication fusion technique, is a technique of wireless communication which loads information on illumination of a light source. Conventionally, this is a technique of receiving light of a light source using a photo diode (PD), detecting digital data of 1 or 0 according to on/off of the light source and transferring information through combination of the digital data.
Conventionally, a visible light communication system is proposed to photograph a plurality of LEDs using a camera, instead of a photo diode, and extract data corresponding to on/off of the LEDs acquired from each frame of the camera. The visible light communication system using a camera like this is also referred to as an Optical Camera Communication (OCC) system since it uses a camera, rather than a photo diode, as an optical receiver, and a standardization work is under progress by the IEEE 802.15.7a study group.
Recently, there is an attempt of applying a rolling shutter camera to such an optical camera communication (OCC) system. The rolling shutter camera uses an electronic shutter employed in an image sensor, which acquires an image of each frame by combining images captured at each row of image sensors arranged in a plurality of rows. Since data transmission can be performed by turning on/off the LEDs in correspondence to a data to be transmitted and restoring the data by analyzing and processing the images captured by the image sensors, it is named as Image Sensor Communication (ISC).
However, in the conventional technique, a technique of extracting a data corresponding to on/off images of a light source of each row using the rolling shutter camera is not clearly presented yet, and since photographing begins at an arbitrary time point when the image sensor communication system photographs on/off images of a light source using the rolling shutter camera, there are occasions in which frames of the rolling shutter camera are not accurately synchronized with on/off timing of the light source. In this case, there is a problem in that it is difficult to extract accurate data.
In addition, although the frame rate of a conventional general rolling shutter camera is fixed to 30 fps, the frame rate actually changes in a range of 20 to 35 fps according to products. Therefore, when the pulse rate of an LED is constant, change of the frame rate of the camera may invite loss of data. For example, if the camera operates when the frame rate of the rolling shutter camera is unstable and changes or when change of the frame rate is unexpected, there is a problem in that data loss may occur since the camera does not photograph an image when the LED is turned on/off between two image frames.
In addition, the conventional technique has a problem in that it is difficult to extract accurate data since the frame of the rolling shutter camera is not synchronized with on/off timing of the light source as the rolling shutter camera begins photographing at an arbitrary time point, and furthermore, since strength of a transmitted signal is weak and on/off images of the LED cannot be clearly distinguished if the distance between the LED, i.e., a transmitter, and the rolling shutter camera, i.e., a receiver, is long, it is difficult to use the image signal strength of LED pixels in long distance transmission.
Furthermore, conventionally, when a plurality of diverse rolling shutter cameras having frame rates and sampling rates different from each other performs image sensor communication, there are occasions in which several rolling shutter cameras do not receive the same data from the same LED.